fix mypy

Author: brokkoli71

tests/test_array.py

@@ -883,7 +883,7 @@ async def test_nbytes(
         assert arr.nbytes == np.prod(arr.shape) * arr.dtype.itemsize
 
 
-async def test_scalar_array():
+async def test_scalar_array() -> None:
     arr = zarr.array(1.5)
     assert arr[...] == 1.5
     assert arr[()] == 1.5